The conventional image forming optical system used as an image transmission unit for optical writing systems such as photo-printers, or for optical reading systems such as scanners often incorporates a rod lens array comprising rod lenslets each having a different refractive index. The erect image optical system incorporating such a lens array can be very compact in size. However, for the manufacture of such a lens array, it is necessary to arrange a large number of rod lenslets in a regular pattern, and to fix them using a resin. Thus, regular arrangement of the rod lenslets is often disturbed, that is, an uneven arrangement of the lenslets may occur. Such an uneven arrangement of the lenslets would affect the resolving power of the lens array and, if such a lens array is incorporated in a high-resolution image forming system (having, for example, a resolution as high as 1200 dpi) as is available on the recent market, will cause the formation of an uneven image.
To avoid the occurrence of such uneven arrangement of lenslets, a proposal to substitute the rod lens array for an erect image resin lens array was presented. The erect image resin lens array is an erect image optical system obtained by arranging a large number of lenslets on a transparent planar substrate, to produce thereby a resin lens plate, and by stacking two or more such resin lens plates one over another to give a layered assembly (lens array). Because fabrication of the resin lens plate is based on the injection of a resin into a die, the precision with which lenslets are arranged as designed is very high, and fabrication of large-sized resin lens plates is comparatively easy. Moreover, it is also possible to arrange lenslets so densely as to eliminate gap, which might otherwise exist between adjacent lenslets, which will contribute to the construction of an erect image lens array capable of transmitting incident light highly efficiently.
However, because fabrication of the aforementioned resin lens plate usually must include arranging lenslets on a transparent planar substrate, out of rays incident to a given lenslet, a certain fraction, distinct from the remaining fraction which is responsible for the formation of an image, passes through the lens body to enter adjacent lenslets to become stray light there. To prevent the occurrence of stray light, a number of countermeasures have been developed: a light-shielding strip is applied along the peripheries of each lenslet excepting its lens portion in contact with the substrate, or, when plural resin lens plates are stacked to give a layered assembly (lens array), shielding septa are introduced between adjacent resin plates at appropriate positions. However, because stray light includes a component passing through the lens body itself, shielding all the other parts than the lens body of each lenslet is not yet sufficient for completely eliminating the stray light component out of the light incident to the lenslet. As far as the lens array is concerned, if a given stack of lenses are shielded with inter-layer septa as well as with individual periphery covering films, the entry of rays other than those incident nearly perpendicularly to the surface of the lens stack will be nearly completely rejected. However, addition of such a structural element as septa to the lens array will increase the overall volume of the array itself, which will be undesirable in many applications.
To date, the erect image resin lens array has been used, in combination with a liquid crystal element, for projecting an image reproduced on a liquid crystal display into a space or on the surface of an object. For said array to be successfully applied for a photo-printer or a scanner, it is necessary to improve the light transmission of each stack of lenslets as well as the resolution of the array. In addition, if the lens array serves as an optical system to receive so-called scanning traces, it must have an elongated form.